Preparation of thermoplastic sheets



l atentetl lain. lttl, ltlilt stares LET H. EAJEIKELAND, 01? YOZNKERS, NEW "YORK, ASSIGNOR T BAKELITE CORPORATION, OF NEW YORK, 'N. Y., A CORCPDRATION OF DELAWARE PREPARATIEN Uh THERMOPIEASTIG SHEETS This invention relates to the production in continuous operation and in sheet form of substantially dry thermoplastic mixtures of a fiber (organic or inorganic) and a synthetic resin of the reactive type. Reactive resins, as is now well understood in this art, are such as are transformed by sufficient application of heat into a hard, strong and infusible state, generally designated as condition C. is

according to the present invention, the re active resin and fiber may first be beaten together in presence of water, and the resulting pulp laid into sheets, in known manner, these operations and the equipment used being somewhat similar to those employed in manufacture of paper. The excess water is removed on the screens by suction or other wise as in paper making.

in the manufacture of ordinary paper the residual water is readily driven out by passing the yet moist sheet over steam heated rolls, this operation also conditioning the sizing and developing the desired qualities in the paper. But in the production of thermoplastic sheets as herein contemplated, a totally different situation arises, in thatthe water, or a large portion of it, must be elimihated without materially affecting or injuring 39 the thermoplastic or other characteristics of the resin binder. This result cannot be accomplished by the use of the usual equipment of the paper mill: nevertheless it is essential that the elimination of water should take piace rapidly and under conditions affording continuous and large scale production at minimum cost, While leaving the dried'sheet in the most serviceable condition for its ultimate applications. Such ultimate applications involve compacting the sheet with or without previous shredding, into such various shapes as plates, slabs, blocks, tubes, gears, and special articles of all kinds, reuuiring thermoplastic condition of the binder resin. 7

it preferred method of accomplishing this result in accordance with the present invention includes subjecting the moist sheet to a W stream of dehydrated air or other gas under conditions adapted to effect the rapid re- Application filed July 5, 19538.

Tn the manufacture of thermoplastic sheets Serial No. 290,702.

moval without injuriously advancing the resin towards condition C or affecting its plasticity. This is best accomplished by first artificially dehydrating the air or other gas either by chemical dehydrating agents such as concentrated sulfuric acid, calcium chlorid or the like; or by physical means, such as refrigerating devices through which the air is directed, preferably in contact with the surfaces of pipes or coils cooled by circulating brine, ammonia or sulfur dioXid or similar refrigerating agents to the point that water vapor content is precipitated as cold Water, ice or snow. The dehydrated gas is then heated, as for example by passing it through a bank of steam coils, and is blown upon or over the moving sheet in suitable chamhers, tunnels or conduits. It is advantageous to blow the gas against both upper and lower surfaces of the moving sheet, especially if the latter is of considerable thickness. It is likewise advantageous to direct the gas current upon the middle portion of the sheet in such manner that the drying proceeds from the middle outwardly, asany tendency to curling at the edges is thereby minimized.

lit has been found that the liability to injury of the sheet is greater while the r0 ortion of moisture present in the sheet 1s higher; so that somewhat more freedom in the ap plication of heat is permissible as the drying progresses. The temperature of the air or gas may vary considerably according to particular conditions, including the thickness and rate of movement of the sheet, the percentage of moisture therein, the velocity of the gas stream, the special characteristics of the resin binder with respect to plasticity and rapidity of reaction, the particular characteristics desired in the dried sheet, etc. etc. The maximum temperature which maybe safely used under any given conditions and for the production of any given product is however readily determined by simple test.

It is important that the actual temperature of the sheet should be kept below the fusing or excessive softening temperature of the resin binder, since fusion or excessive softening is apt to lead to sticking of the sheets and the formation of impervious areas, spots and other defects. It will be understood, however, that the absorption of heat by evaporation of the moistue tends strongly to cool the sheet, so that so long as evaporation is sufiiciently rapid, the actual temperature of the dehydrating gaseous medium may be decidedly above the melting point of the resin Without injury to the latter. For this reason no definite limitation is placed upon the temperature of the gas, while the temperature of the moving sheet is carefully controlled.

In special cases gaseous media other than air, for example, inert or non-oxidizing gases such as nitrogen, combustion gases, etc. may be used. If desired, the air or gas may be circulated in a closed cycle passing alternately and repeatedly in contact with the moving sheet and the dehydrating means.

The extent to which the drying of the sheet is carried in any particular case will of course depend upon the attending circumstances. In general, absolute dehydration of the sheet is not necessary at this stage, since some reabsorption of moisture may take place in any prolonged period of subsequent storage, unless special precautions are adopted.

The present process is applicable to mix- .tures containing reactive synthetic resins of all types, although more specially intended for use with reactive resins of the known phenol aldehyde and cresol aldehyde types.

I claim: 1. Process of preparing a substantially dry fibrous sheet containing a synthetic resin in reactive form which comprises the steps of preparing an aqueous suspension of fiber and reactive resin, continuously laying the mixture into a sheet and draining excess water therefrom, dehydrating a current of gas, heating said current of dehydrated gas, and applying said current of heated dehydrated gas to the moving sheet at a rate and a temperature such that thetemperature of the sheet remains below that at which excessive softening of the contained resin occurs whereby residual moisture is rapidly removed without substantially injuring the reacting properties of the resin or modifying its plasticity.

2. Process of preparing a substantially dry fibrous sheet containing a synthetic resin in reactive form which comprises the steps of preparing an aqueous suspension of fiber and reactive resin, continuously laying the mixture into a sheet and draining excess water therefrom, dehydrating a current of gas, heating said current of dehydrated gas, and passing said current of heated dehydrated gas over at least one surface of the moving sheet at a rate and a temperature such that the temperature of the sheet remains below that at which excessive softening of the contained resin occurs whereby residual moisture 1s rapidly removed without substantial- 1y injuring the reactin properties of the resin or modifying its p asticity.

3. Process of preparing a substantially dry fibrous sheet containing a synthetic resin in reactive form which comprises the steps of preparing an aqueous suspension of fiber and reactive resin, continuously laying the mixture into a sheet and draining excess water therefrom, dehydrating a current of gas, heating said current of dehydrated gas, and passing said current of heated dehydrated gas over both surfaces of the moving sheet at a rate and a temperature such that the temperature of the sheet remains below that at which excessive softening of the contained resin occurs whereby residual moisture is rapidly removed without substantially injuring the reactin properties of the resin or modifying its p asticity.

4. Process of preparing a substantially dry fibrous sheet containing a synthetic resin in reactive form which comprises preparing an aqueous suspension of fiber and reactive resin, continuously laying the mixture into a sheet and draining excess water therefrom, dehydrating a current of gas, heating said current of dehydrated gas, and passing said current of heated dehydrated gas over at least one surface of the moving sheet, the gas current being directed upon the middle portion thereof at a rate and a temperature such that the temperature of the sheet remains below that at which excessive softening of the contained resin occurs whereby residual moisture is rapidly removed without substantially injuring the reacting properties of the resin or modifying its plasticity, and whereby the drying of the sheet proceeds from the middle outwardly.

5. Process of preparing asubstantially dry fibrous sheet containing a synthetic resin in reactive form which comprises the steps of preparing an aqueous suspension of fiber and reactive resin, continuously laying the mixture into'a sheet and drainin excess water therefrom, heating a current 0 gas, and passing said current of heated gas over at least one surface of the moving sheet at a rate and a temperature such that the temperature of the sheet remains below that at which excessive softening of the contained resin occurs whereby residual moisture is rapidly removed without substantially injuring the reacting properties of the resin or modifying its plasticity.

In testimony whereof, I aflix my signature.

LEO H. BAEKELAND. 

